The purpose of this study is to evaluate the variable pressure correction technique (VPCT) as a solution to the problem of extraneous x-ray peaks due to electron beam broadening in the chamber gas of the ESEM. The basis of VPCT is the observation that target x-ray counts decrease with increasing chamber pressure; whereas, x-ray counts due to beam broadening increase. If data are collected at two or more chamber pressures, the number of x-ray counts for an element can be corrected to that expected at zero gas pressure (high vacuum). Tests of NIST SRM 482 have shown EDS x-ray analysis in the ESEM (within the chamber pressure range of 1 to 8 torr) to have comparable accuracy and precision values to those of EDS in the traditional SEM. The samples used in the these studies, however, were quite large (ca. 500 μm in diameter) and so extraneous EDS peaks, due to the electron beam broadening effect of the chamber gas, were minimized.The 60% Au / 40% Cu wire of SRM 482 was pressed into a hole in the surface of an Al specimen stub so as to produce a flat surface with a sharp interface between the wire and the stub. Spectra were collected at 5 and 150 μm from the junction of the wire and the Al stub at chamber pressures of 2, 4, and 8 torr.
Experimental determination of electron-beam broadening in low-energy STEM